The above would replace all of the rcParams that the inline backend sets, and you get total control. If you already have a matplotlib style that works nicely for inline output, you can tell the backend to leave everything alone:

c.InlineBackend.rc = {}

If you want to change just a few values, rather than overriding the whole thing, you can use the dictionary .update method:

c.InlineBackend.rc.update({'figure.figsize': (15, 9)})

In the future, the inline backend should be doing its defaults via matplotlib's nice new style mechanism, which should make it behave nicer in terms of respecting your preferences and allowing easier customization.